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@yuxianq yuxianq commented Jan 24, 2026

Summary by CodeRabbit

  • New Features
    • Added multi-stream sampling support for distributed pipeline parallel configurations, optimizing sampling performance with asynchronous execution on dedicated streams via the TRTLLM_PP_MULTI_STREAM_SAMPLE environment variable.

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…ream in multi-stream cases.

Signed-off-by: Yuxian Qiu <142763828+yuxianq@users.noreply.github.com>
@yuxianq yuxianq requested a review from litaotju January 24, 2026 06:14
@yuxianq yuxianq requested a review from a team as a code owner January 24, 2026 06:14
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yuxianq commented Jan 24, 2026

/bot run --disable-fail-fast --add-multi-gpu-test

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PR_Github #33436 [ run ] triggered by Bot. Commit: 18516aa

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coderabbitai bot commented Jan 24, 2026

📝 Walkthrough

Walkthrough

This change adds multi-stream sampling support to PyExecutor by introducing the pp_multi_stream_sample configuration flag, importing TRTLLMSampler, and creating a dedicated CUDA stream with event synchronization for asynchronous sampling operations when pipeline parallelism is enabled.

Changes

Cohort / File(s) Summary
Multi-stream sampling initialization and execution
tensorrt_llm/_torch/pyexecutor/py_executor.py
Added TRTLLMSampler import; introduced pp_multi_stream_sample boolean flag configurable via environment variable; added dedicated sample_stream, start_sample_event, and finish_sample_event creation; implemented conditional sampler store/algorithm reinitialization on separate stream when applicable; modified executor loop to use flag instead of direct environment variable check; updated sampling path to clone batch_outputs, synchronize via events, and perform asynchronous sampling on dedicated stream

Sequence Diagram(s)

sequenceDiagram
    participant Forward as Forward Pass<br/>(Main Stream)
    participant Events as Event<br/>Synchronization
    participant SampleStream as Sample Stream
    participant Sampler as TRTLLMSampler

    Note over Forward,Sampler: Multi-Stream Sampling Flow (PP mode, enabled)
    
    Forward->>Forward: Execute forward computation
    Forward->>Events: Record start_sample_event
    Forward->>Events: Signal sampling can begin
    
    par Forward and Sampling in Parallel
        Forward->>Forward: Continue next iteration
    and
        SampleStream->>SampleStream: Wait for start_sample_event
        SampleStream->>SampleStream: Clone batch_outputs
        SampleStream->>Sampler: Sample on dedicated stream
        Sampler->>Sampler: Generate next tokens
        SampleStream->>Events: Record finish_sample_event
    end
    
    Forward->>Events: Wait for finish_sample_event
    Forward->>Forward: Use sampled outputs
Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

Suggested reviewers

  • litaotju
  • zhenhuaw-me
  • ziyixiong-nv
🚥 Pre-merge checks | ✅ 1 | ❌ 2
❌ Failed checks (2 warnings)
Check name Status Explanation Resolution
Description check ⚠️ Warning The PR description is incomplete. While the title and template are present, critical sections like 'Description' and 'Test Coverage' are empty, leaving the actual motivation and test strategy unexplained. Fill in the Description section explaining the issue and solution, and the Test Coverage section listing relevant tests that validate the changes.
Docstring Coverage ⚠️ Warning Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (1 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly summarizes the main change: re-initializing the TRTLLM sampler to use sample stream in multi-stream cases, with proper ticket format and type indicator.

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Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (3)
tensorrt_llm/_torch/pyexecutor/py_executor.py (3)

1-1: Add NVIDIA copyright header (latest year).
This file lacks the required NVIDIA copyright header for TensorRT-LLM sources; please add/update it to reflect the latest meaningful modification year. As per coding guidelines, please add the standard header at the top of the file.


1107-1124: Gate multi-stream sampling to TRTLLMSampler for consistency with initialization.

The sampler re-initialization at line 260 is correctly gated to isinstance(self.sampler, TRTLLMSampler), but the multi-stream sampling execution at line 1107 is only checked against pp_multi_stream_sample. This creates a mismatch: if a non-async sampler (e.g., EarlyStopSampler) is used with pp_multi_stream_sample=true, the code will attempt stream operations without confirming the sampler is stream-safe. Add the isinstance check:

-                            if self.pp_multi_stream_sample:
+                            if self.pp_multi_stream_sample and isinstance(self.sampler, TRTLLMSampler):

1111-1115: Guard batch_outputs_copy cloning for non-tensor values.

The output dict from model_engine.forward() can contain non-tensor values: None (e.g., 'logits': None in mm_encoder_only mode), lists of tensors, or lists of scalars. Calling .clone() on these will raise AttributeError. Clone only tensor values.

🛠️ Suggested fix
-                                batch_outputs_copy = {
-                                    name: tensor.clone()
-                                    for name, tensor in batch_outputs.items()
-                                }
+                                batch_outputs_copy = {
+                                    name: tensor.clone() if torch.is_tensor(tensor) else tensor
+                                    for name, tensor in batch_outputs.items()
+                                }
🧹 Nitpick comments (1)
tensorrt_llm/_torch/pyexecutor/py_executor.py (1)

56-57: Use module namespace for sampler imports.
Project guidelines require preserving module namespaces (avoid from x import y). Consider importing the module and referencing sampler.TRTLLMSampler / sampler.Sampler, etc. As per coding guidelines, please keep namespace imports.

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PR_Github #33436 [ run ] completed with state SUCCESS. Commit: 18516aa
/LLM/release-1.2/L0_MergeRequest_PR pipeline #216 completed with status: 'SUCCESS'
Pipeline passed with automatic retried tests. Check the rerun report for details.

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